Mixed Phase Orographic Cloud Microphysics during StormVExand IFRACS

Abstract. Wintertime mixed phase orographic cloud (MPC) measurements were conducted at the Storm Peak Laboratory (SPL) during the StormVEx and IFRACS programs in 2011 and 2014, respectively. The data include 92 hours of simultaneous measurements of supercooled liquid cloud droplet and ice particle size distributions (PSD). Average cloud droplet number concentration (CDNC), droplet size (NMD) and liquid water content (LWC) were similar in both years while ice particle concentration (Ni) and ice water content (IWC) were higher during IFRACS. The consistency of the liquid cloud suggests that SPL is essentially a cloud chamber that produces a consistent cloud under moist, westerly flow during the winter. A variable Cloud Condensation Nuclei (CCN) related inverse relationship between CDNC and NMD strengthened when the data were stratified by LWC. Some of this variation is due to changes in cloud base height below SPL. While there was a weak inverse correlation between LWC and IWC in the data as a whole, a stronger relationship was demonstrated for a case study on February 9, 2014 during IFRACS. A minimum LWC of 0.05 g m −3 showed that the cloud was not completely glaciated on this day. Erosion of the droplet distribution at high IWC was attributed to the Wegener-Bergeron-Findeisen process although the high IWC was caused by a 10 fold increase in Ni. A relationship found between large cloud droplet concentration (25–35 µm) and small ice particles (75–200 µm) under cold ( °C) and warm (u003e−8 °C) conditions suggests ice particle production by contact or immersion freezing. Such a relationship under warm conditions could be indicative of biological ice nuclei. There was no direct evidence of secondary ice production. The effect of blowing snow was evaluated by comparing the relative (percent) ice particle PSDs at high and low wind speeds. These were similar, contrary to expectation for blowing snow. However, correlation between wind speed and ice crystal concentration may support this explanation for high crystal concentrations at the surface. Further experimental work is needed to resolve this issue.